As described in pending patent applications such as US2005072732 and US2003127378, various biomass carriers have been designed for use in biological waste water treatment systems. These carriers are contained within a biological reactor vessel and are maintained in a fluidized or constant motion state. Since the biomass carriers are in constant motion, they will collide with each other. Hence the carriers have protected surfaces, typically inside surfaces, that are not exposed to collisions. The reactor vessel receives waste water that may be aerated or oxygenated to support aerobic biological processes useful in the treatment of waste water. The biomass carriers are used to provide an anchorage or substrate to support populations of micro-organisms. These micro-organisms form a bio-film over the protected surfaces of the carriers. The bio-film organisms react with the waste water and remove organic pollutants from the water so that outflow from the reactor vessel has a lower concentration of organic contaminants. One advantage of using biomass carriers in reactor vessels is that the size of the vessel can be reduced.
Biomass carriers are typically designed to maximize the protected surface for maximum bio-film growth while keeping the overall volume of the carrier to a minimum. This has the result of maximizing biological treatment of waste water with an optimally dimensioned carrier. The prior art reveals a wide variety of carrier shapes including extruded plastic shapes with radial fins and/or concentric rings, multi-cellular (foam) shapes and composite shapes consisting of a framework material with another high surface area material attached to the framework. However, there is a practical limit to the ratio of protected surface area to volume in biomass carrier design. Since the biomass carrier relies upon effluent flow through the inside surfaces supporting the bio-film, increasing the volume of these inside surfaces in an effort to maximize biological treatment has the deleterious effect of minimizing flow channels through the biomass carrier thereby reducing the volume of waste water flow through the carrier. The smaller flow channels are prone to clogging and cannot provide sufficient waste water flow over the bio-film for efficient treatment.
The known art also teaches the use of biomass carriers to enhance nitrogen conversion and removal. Biomass carrier systems have been employed for nitrification (conversion of ammonia to nitrate) and de-nitrification (conversion of nitrate to nitrogen gas). Nitrification occurs primarily in aerobic (oxygen-rich) conditions while de-nitrification requires anoxic conditions as well as a substance (such as organic compounds) to act as an electron acceptor.
The accepted chemistry for nitrification and de-nitrification is as follows:NH4++1.5 O2→NO2−+2H++H2O   (1)NO2−+0.5 O2→NO3−  (2)2 NO3+10H++10e−→2 OH−+4 H2O+N2   (3)
Summary: 2NH4++4 O2−+10e−+6H+→2 OH−+6H2O+N2
While nitrification and de-nitrification in wastewater treatment is usually a sequential aerobic/anoxic process with each step implemented in separate reactor vessels or compartments and some form of recycle between the vessels or compartments employed, the art indicates that there have been efforts to stimulate simultaneous nitrification-de-nitrification in biological treatment systems. This is known as SNdN in the waste water treatment industry. A paper by the engineering firm Black and Veatch describes the somewhat unexpected contribution of SNdN to nitrogen removal in an Integrated Fixed-film Activated Sludge (IFAS) system employing biomass carriers (“Pilot Scale Performance of the MBBR process at the Crow Creek WWTP” J. P. McQuarrie and M. Maxwell, WEFTEC 2003). In these “hybrid system” applications, SNdN occurrence has been attributed to the presence of anoxic micro-sites within a generally aerobic environment. SNdN has considerable advantages over the traditional two step sequential nitrification-de-nitrification process because it conserves alkalinity, requires less oxygen and hence less process energy, as shown by the chemistry below:NH4++1.5 O2→NO2−+2H++H2O   (1)2 NO2−+6H++6e−→2 OH−+2H2O+N2   (2)
Summary: 2NH4++3O2+6e−+2H+→2OH−+4H2O+N2
My review of the known art indicates biomass carrier design for optimizing SNdN is wanting. Some efforts have relied upon the use of multi-porous media such as foam to contain anoxic and anaerobic micro-sites even when the waste water contains high levels of dissolved oxygen. However, for a carrier to have effective SNdN the anoxic and aerobic micro-sites must be in close proximity to establish efficient transport of nitrite between the nitrification sites and the de-nitrification sites. SNdN in systems with multi-porous (foam) type media consistently fail to exceed 30% reduction in total nitrogen. One reason is that these foam carriers tend to produce an aerobic outer layer and a massive internal anaerobic layer. The result is limited potential for producing the necessary anoxic sites and severe mass transfer limitations caused by clogging of passages between the aerobic and anoxic zones. In addition, these media have operational complications such as the need to periodically squeeze the biomass out of the foam to restore porosity. As a result of such operational drawbacks, the use of extruded and to a lesser extent, injection-moulded plastic biomass carriers has been much more extensive in the industry.
It should be noted that in the field of biomass carriers, certain commonly used comparative parameters have limitations that can lead to erroneous assumptions about performance. The most commonly cited biomass carrier characteristic is called the “specific surface area”. This is a measure of the total surface area of the carrier per unit volume of the carrier when the carrier is random-packed in a dry state. Derivatives of this parameter include “protected surface area” which deducts areas exposed to carrier-carrier and carrier-vessel collisions where the bio-film would not likely survive. The collision areas are deemed to contribute little to the overall performance of the biomass carrier. Dry packing efficiencies vary with carrier design. Carrier performance is best indicated by the total protected surface area per volume of bioreactor. The total surface area of a carrier is determined by the maximum fill fraction of the carrier that is consistent with adequate carrier motion and circulation in the bioreactor. Some commercial biomass carriers have a maximum fill fraction of about 65-70%. With protected surface area values of 400 to 500 square meters per cubic meter this yields a maximum in-service specific protected surface area of 260 to 350 square meters per cubic meter. Considering the wide range of biomass carrier designs, these maximum in-service specific protected surface area values are not necessarily proportional to dry-packed specific surface area values. The protected surface area values are influenced by hydro-dynamic characteristics within the reactor such as carrier interactions with air bubbles, tendencies for carriers to “bridge” and other factors too complex to predict accurately with available hydro-dynamic and mixing models.
For a biomass carrier to be effective for SNdN, the bio-film it supports must have aerobic and anoxic sites in close proximity for the efficient transfer of nitrites between them. The levels of dissolved oxygen in waste water required for SNdN will be lower than is optimal for nitrification and higher than is optimal for de-nitrification. SNdN has been observed to be considerably more prominent and consistent in biomass carrier systems operating in hybrid Integrated Fixed-film Activated Sludge (IFAS) mode. These have achieved approximately 40% reduction in total nitrogen. In these hybrid systems large concentrations of suspended biological flocs (typical of the Activated Sludge process) coexist with the biomass carriers and compete for oxygen and substrate. It has been observed that populations of nitrifying bacteria are higher as a proportion of total flora in the bio-film compared with the suspended populations. It is thought that in these hybrid systems, anoxic sites are more prevalent than in pure biomass carrier systems because of the lower dissolved oxygen levels in the region of the bio-film due to competition from suspended biomass and the greater degree of bio-film masking by suspended particulate matter. SNdN occurrence in these hybrid applications has been observed to be inconsistent and not reliable for process design purposes. Further, it suffers from the requirement for sub-optimal dissolved oxygen levels, resulting in an excessive sacrifice of primary treatment performance.
Bio-film growth and morphology is complicated and affected by a large number of factors. However a few general principles apply:                (1) Bio-film thickness is generally positively correlated with the specific organic loading rate, that is, the mass of organic matter applied to the bioreactor per total protected surface area of carriers; bio-films in applications with low loading rates and predominantly lower metabolic potential (such as separate stage nitrification) are noted to exhibit particularly thin bio-films;        (2) Anoxic micro-sites are more likely to occur in thicker bio-films and will be preferentially located closer to the carrier surface;        (3) Bio-films tend to “smooth out” surface irregularities, prominences, crevices and angles that are of a scale comparable to or smaller than that of the biomass thickness; and,        (4) Bio-films in more protected areas will on average be thicker than those in more exposed locations. For example, areas exposed to carrier-carrier collisions will be very thin because of the physical attrition effect. Similarly, bio-films in areas exposed to lower bulk fluid velocities will on average be thicker than those in areas exposed to high velocities. This is partly due to the fact that bio-film sloughing is encouraged by high velocities.        
To date, biomass carriers as well as other secondary treatment technologies have been employed for the removal of generalized organic content measured by biochemical oxygen demand (BOD) and chemical oxygen demand (COD) as well as nutrients such as various species of Nitrogen and Phosphorus. Increasingly, attention is focussed on a new class of pollutants known as “trace organic contaminants” such as pharmaceutical residuals. The ability to address these contaminants will be crucial in future waste water treatment systems. Recent studies have recognized that removal of these contaminants is considerably more efficient in a biological system with a high sludge age and heterogeneous populations.
Therefore there is a requirement for an improved biomass carrier-based system of waste water treatment that can consistently produce similar or better nitrogen removal efficiencies as has been noted to be achieved (episodically) by hybrid systems. The new method should preferably not rely on the presence of suspended biological matter. There is also a requirement for an improved system of waste water treatment that is adapted for the removal of trace organic contaminants.